This invention relates generally to the elimination of polluting constituents from gaseous streams and is particularly directed to the removal of solid particulates from a high temperature gas.
The prior art discloses various approaches for treating waste gases to separate, for example, fly ash from flue gases. The terms fly ash, particulates, particles, and dust are used herein interchangeably. These approaches typically make use of gas filters, dynamic air-solids separators, wet scrubbers and electrostatic precipitators. All of these prior approaches suffer from various limitations and few of them are capable of effective removal of gas phase impurities from gaseous streams.
Bag filters or fiber fabric filters which have proved effective in removing solid contaminants from gaseous streams tend to become clogged with the gas entrained particles resulting in an excessive pressure drop across the filter, necessitating the use of some means to clean the surface of the filters. One approach for cleaning filters of this type involves the use of short pulses of pressure in a direction reverse to the normal gas flow to dislodge the particles entrapped by the filter. Another technique utilized to remove gas entrained particles involves the passing of contaminated gas through a bed of granular material such as sand. The gas entrained particles are collected on the upstream surface of and within the sand bed. Fixed beds suffer from the same disadvantage as the fiber or porous type filters discussed above, i.e., gas flow must be stopped for cleaning. Fluidized beds are continuous in operation but are less effective in removing small particles than a fixed or dense packed moving bed. In addition, sudden surges of gas passing through the fluidized bed can result in not only the previously contained particles passing through the bed, but also a portion of the bed itself being entrained in the gas, thus adding even more to the particulate loading of the gas stream.
The moving bed of granular material serves not only to remove particulates from the gas stream, but also removes gas entrained particulates from the filter through which the gas is directed. It is in this manner that prior art filter approaches have attempted to maintain the filter in a clean condition and free from clogging particulates. Unfortunately, contact of the moving bed of granular material with the filter causes excessive wear of the filter, particularly for the more common woven cloth filters. Hence, filtering arrangements which make use of a moving bed of granular material require frequent replacement of the filter element.
Over the past twenty-five years, attempts to reduce air pollution led to the use of copper oxide for removing SO.sub.X from flue gas or copper oxide in combination with ammonia for simultaneous removal of NO.sub.X and SO.sub.X from flue gas. Copper oxide reacts with SO.sub.X forming CuSO4. The NO.sub.X is catalytically reduced with ammonia, forming nitrogen and water. The absorber-catalyst is regenerated by reacting the copper sulfate thus formed with a reducing gas such as hydrogen, methane, or synthesis gas, and is then reused in the process. The regeneration step produces a side stream of concentrated SO.sub.2, which can be converted to liquid SO.sub.2, elemental sulfur, or sulfuric acid by known processes. The optimum temperature for the SO.sub.X reaction, the NO.sub.X reduction, and the catalyst regeneration all occur at approximately 400.degree. C. The process is regenerative, dry, and produces no waste materials.
Several different types of contactors have been proposed for copper oxide processes. To avoid the plugging of fixed beds by soot and fly ash particles, an "Open Channel" reactor has been developed, in which the flue gas is passed alongside large surfaces of absorbent mass rather than through a particle bed. With this parallel passage design, soot and fly ash pass through the channels without plugging the absorbent bed. The choice of the open channel reactor results in a cyclic process. This process uses two reactors. While one is absorbing SO.sub.2, the other is regenerated. The cyclic flow rates in the regenerator are dampened with gas holders and an absorber-stripper. This process is known as the Shell/Union Oil Products (UOP) process.
Another approach being developed employs a fluidized-bed copper oxide process for coal-fired combustion sources wherein the fly ash passes through the fluidized bed without plugging it. This process has the advantage that it is continuous and offers intimate contact between the gas and the catalyst. However, this approach as well as the Shell/UOP "Open Channel" reactor discussed above requires a separate downstream particle collector to control the fly ash on coal-fired power plants.
The present invention overcomes the aforementioned limitations of the prior art by providing a single-stage, continuous process for simultaneously removing 90% of the NO.sub.X, 90% of the SO.sub.X, and 99.9 of the particulate material from flue gas from coal-fired combustion sources. Apparatus in accordance with the present invention utilizes a novel contactor/filter with a moving-bed of granular copper oxide absorber-catalyst which is retained between louvers or a screen on the gas inlet side and a sheet of filter material on the exit side. A cover screen is disposed between the upstream side of the sheet of filter material and the moving-bed of copper oxide absorber-catalyst.